libfuse: don't force -D_FILE_OFFSET_BITS=64 in pkgconfig file.

libfuse: don't force -D_FILE_OFFSET_BITS=64 in pkgconfig file.

FUSE_CFLAGS defines -D_FILE_OFFSET_BITS=64.  There are three problems
with this:

(1) A larger program using libfuse might have modules compiled with
and without FUSE_CFLAGS, which, if LFS is not enabled and the platform
is 32 bit, would result in a fatal mix of 32 and 64 bit off_t.  (This
would, of course, be a bug, but I think there is a better way to
detect this -- see below)

(2) Programs may need to be adjusted to support LFS.  It's the
intention of the LFS standard that the _programmer_ enables LFS once
the program has been checked/adjusted.

(3) _FILE_OFFSET_BITS does not need to be defined at all on 64 bit
Linux.  64 bit off_t is the default there.

So I think it's better not to force -D_FILE_OFFSET_BITS=64, and
because of (3) I also think you shouldn't test for it.

However off_t must still be 64 bits, so how to enforce that?  C1X will
define static assertions[1], and these can be used to check the size
of off_t.

Not all compilers support static assertions yet, although several do.
Therefore I have surrounded the static assertion with a conservative
check that the compiler is GCC >= 4.6.  In the long run, this test can
be removed and you can just use 'static_assert'.